In some tubing strings, it may be necessary to drill out certain obstructions. For example, a selective fracing tool such as a tool described in U.S. Pat. No. 7,431,091 (Themig et al.) entitled “Method and apparatus for wellbore fluid treatment” uses different sizes of balls to selectively close sections of the tubing string to apply fracing pressure to a desired section of a formation. The balls and preferably some or all of the ball seats must then be removed in order to allow fluid to flow freely once the facing operation has been completed. Other tubing strings may have other components for similar or different operations, may be filled with cement that must be removed, or may become filled with debris such that a cleanout operation is required. These various operations are generally performed by a thru-tubing mill.